As a method used at a resist stripping step in semiconductor fabrication, a sulfuric acid electrolytic method has been known wherein sulfuric acid solution is electrolyzed to form peroxosulfuric acid (peroxodisulfuric acid: molecular peroxosulfuric acid and ionic peroxosulfuric acid) and then cleaning is performed using the peroxosulfuric acid solution as cleaning fluid. Resist stripping efficiently proceeds at a resist stripping step, when the temperature of cleaning fluid is elevated (to about 120° C. to 190° C.). It can be considered that this is because when the temperature of cleaning fluid produced using such a sulfuric acid electrolytic method is increased to a predetermined high temperature, peroxosulfuric acid in the cleaning fluid autolyzes, and sulfate radicals having extremely strong oxidation power are thus generated and contribute to cleaning.
Radicals have only short lifetime, and therefore, if the temperature of cleaning fluid is raised earlier, peroxosulfuric acid in the cleaning fluid will autolyze too early and its radicals will be consumed without contributing to cleaning. When the temperature of peroxosulfuric acid solution is increased to high temperature, peroxosulfuric acid autolyzes to generate sulfate radicals, and the concentration of sulfate radicals consequently increases. The generated sulfate radicals simultaneously decompose to decrease the concentration of sulfate radicals. The concentration of sulfate radicals peaks after several tenths of a second to several seconds from the elevation of the temperature of peroxosulfuric acid solution, the time interval varying also based on the temperature of the solution. Therefore, since it is most efficient to set the timing of elevating temperature in such a manner as to contribute to cleaning just when the concentration of sulfate radicals reaches its peak, it is necessary to appropriately set optimum timing.
If cleaning fluid is slowly heated over a long time (for example, for several minutes), the autolysis of peroxosulfuric acid and the associated decomposition of sulfate radicals proceed during the elevation of temperature. Therefore, at the time of the completion of the elevation of temperature, the concentration of peroxosulfuric acid is already low. Results shown in FIG. 7 are obtained by theoretically calculating based on reaction kinetics and Arrhenius equation, and the results show that lifetime of peroxosulfuric acid is extremely short under high temperature.
As seen from the above, it is necessary to perform the elevation of temperature of cleaning fluid in a very short time (in several seconds) immediately before cleaning.
On the other hand, under lower temperature, the efficiency of the electrolysis of sulfuric acid solution is higher and the rate of the autolysis of peroxosulfuric acid is lower. It is thus preferable that sulfuric acid solution be electrolyzed at low temperature (about 20° C. to 60° C.). When used as cleaning fluid at a resist stripping step, it is necessary to instantaneously elevate sulfuric acid solution, which is electrolyzed at low temperature, to high temperature from low temperature immediately before cleaning.
Various apparatus for heating fluid have been proposed.
For example, such a fluid heating apparatus 40 as shown in FIG. 8 has been conventionally used in a step of heating pure water or the like in semiconductor fabrication. The fluid heating apparatus 40 includes: a closed quartz vessel 41 in the shape of a tube; a liquid inlet 41a and a liquid outlet 41b provided obliquely with each other on a side wall of the closed quartz vessel 41; and an infrared heater 42 in the closed quartz vessel 41. Pure water or the like flows into the closed quartz vessel 41 through the liquid inlet 41a, is heated by being contacted with the circumference of the infrared heater 42, and then discharged out of the liquid outlet 41b. 
In addition, a fluid heating apparatus 50 as shown in FIG. 9 is known. The fluid heating apparatus 50 includes a double-tube structure. Liquid to be heated flows via an inlet 51a for the liquid to be heated and an outlet 51b for the liquid to be heated both provided for an inner tube 51. On the other hand, heat transfer oil flows through between the inner tube 51 and an outer tube 52 via an inlet 52a for the heat transfer oil and an outlet 52b for the heat transfer oil both provided for the outer tube 52. The fluid to be heated is heated due to the heat exchange between these fluids through a wall of the inner tube 51.
Moreover, a fluid heating apparatus has also been proposed in which heating efficiency is improved by providing a flow channel for fluid to be heated along both outer and inner circumferences of a tubular ceramic heater (see Patent Document 1).